Serial video interfaces described by various SMPTE standards, such as SMPTE 259M, 292M and 424M employ data scrambling using a fixed polynomial. Certain combinations of data input to the scrambling polynomial are known to produce pathological conditions on the serial link. The pathological condition is characterized by long runs of low-frequency patterns.
The low-frequency patterns can cause non-optimum performance in receive devices which employ cable equalization and clock and data recovery, resulting in data errors or failure to recover the original data. The pathological condition can result in increased jitter which produces errors caused by poor clock recovery. The condition can also result in “false locking” since phase-locked loops (PLLs) may lock to the lower-frequency signal produced. Low-frequency data patterns also create DC offsets, requiring the signal to be “DC-restored” at the receive-end. The DC restoration process may add unwanted jitter, reducing timing margin.
The input to the data scrambler is usually in the form of 10-bit parallel data words. Certain combinations of these data words are known to cause the pathological condition. SMPTE RP 198 defines video checkfields which will produce long sequences of the pathological condition, thus providing a stressful “test pattern” for serial digital video systems. The 10-bit data word combinations are converted to serial, least significant bit first, and clocked into the scrambling polynomial.
Within the broadcast and video industry, many concerns have been voiced about the propagation of the “pathological problem” for new serial video interfaces, specifically high-bandwidth interfaces over optical fibre. Standard optical equipment used in the data communications industry is not designed to handle the stressful conditions produced by pathological signals. While companies such as Gennum Corporation have designed components specifically to handle pathological conditions, it is necessary to propose solutions for preventing pathological conditions, in order to get wider adoption of higher data rate interfaces.